Inspired by the gang over at Orion's Arm. The text is mined from Wikipedia though, tweaked to use in a game.
A launch loop, or Lofstrom loop (after the pre-Nanotech age originator of the concept), is a design for a dynamic, continuous belt based, magnetic levitational orbital launch system. It is essentially a hybrid of the orbital ring concept and the space fountain arranged to form a mag-lev acceleration track suitable for launching humans into space. Launch loops provide a way for non-rocket spacelaunch of vehicles weighing 5 metric tons by electromagnetically accelerating them so that they are projected into Earth orbit or even beyond. This would be achieved by the flat part of the cable which forms an acceleration track above the atmosphere.
If a launch loop was built on Earth, it would be aproximately 2,000 km (1,240 mi) long, having a maximum altitude of up to 80 km (50 mi), with two 28 kilometer diameter base stations that can launch, catch and turn around a very fast moving magnetic belt, called a rotor. The actual size of loops varies based on the planetary diameter and gravity. The loop would be held up at its altitude by momentum of the belt as it circulates around the structure, effectively transfering the weight of the structure onto magnetic bearings at each end which support it.
Although the overall loop is very long, at around 4,000 km circumference, the belt itself is thin, around 5 cm diameter and the sheath is not much bigger. The rotor for the loop is made of ferromagnetic iron and is in the shape of a pipe, but with lengthwise expansion joints every metre or so. Higher tech version take advantage of advanced materials, usually super conductors and better insulators. The rotor is spaced from an air-tight sheath by magnetic bearings. The sheath maintains a vacuum which avoids atmospheric friction.
The loop starts off at the planetary surface, and stationary. The rotor is spun up, turned by a several hundred megawatt linear motor (typically powered by a dedicated power station). As the speed increases the central parts of the structure push upwards and curve into a near arch shape. The weight of the structure is held there by the curvature of the rotor which creates a reactive centrifugal force. As the rotor is curved more quickly downwards than a purely ballistic trajectory it applies a force upwards on the cable sheath which holds the sheath aloft. When the cable reaches an altitude of around 80 kilometers the loop is restrained and shaped by cables that hang down to sea level. The rotor would be spun up to a linear speed of 14 km/s (8.7 mi/s). Using a 300 MW power generator, this would take about two months to reach full speed. Once fully commissioned, the rotor would take almost 5 minutes to make a complete circuit of the loop.
Once raised, the structure still needs additional power to deal with power lost in the magnetic bearings, stabilize the structure, and deal energy with losses due to the imperfect vacuum in the sheath; overall this requires around 200 MW per day. Additional energy would be needed to power any vehicles that are launched.
To launch, vehicles are raised up on an 'elevator' cable that hangs down from the West station loading dock at 80 km, and placed on the track. The payload applies a magnetic field which generates eddy currents in the fast-moving rotor,. This both lifts the payload away from the cable, as well as pulling the payload along with 3g (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity, and leaves the track.
If a stable or circular orbit is needed, once the payload reaches the highest part of its trajectory then an on-board rocket engine ("kick motor") or other means is needed to circularize the trajectory to the appropriate Earth orbit.
The eddy current technique is simple, compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 kelvins due to power dissipation. If launches are spaced too close together, the rotor temperature can approach 770 °C (1043 K), at which point the iron rotor loses its ferromagnetic properties and rotor containment is lost.
Launch loops launch at high rates (many launches per hour, independent of weather), and this rate can be increased with materials that remain magnetic at higher temperatures. They are inherently non-polluting, though poor power plant designs or disasters with dangerous cargoes can change this. As a form of electric propulsion can be clean, and can be run on geothermal, nuclear, wind, solar or any other power source, even intermittent ones, as the system has huge built-in power storage capacity.
Unlike space elevators which are subjected to the risks of space debris and meteorites along their whole length, launch loops are at an altitude where orbits are unstable due to air drag. Since debris does not persist, it only has one chance to impact the structure. Whereas the collapse period of space elevators is expected to be of the order of years, damage or collapse of loops in this way is expected to be rare.
Another advantage of launch loops over space elevators, is that they don't have to travel through the Van Allen belts over several days. Launch loop passengers can be launched to low earth orbit, which is below the belts, or through them in a few hours.
Tech Level: Stringtech 3, Nanotech 3 for initial possible manufacture. String 4+ for realistic and safer power, Nanotech 4+ for superconductors.